Loading 44 mag with trail boss...what am I missing?

[JonB_in_Glencoe]

.430 then .428 in 1/8" ? that's not good.

Did you measure more than one J-word?
Since you have some seat easier than others, maybe that is where you variation lies?

You haven't mentioned if you are expanding the case, which is different then just flaring the mouth. Expanding the entire area where the bullet/boolit seats to is what makes the inside uniform for little or no variation in case tension.

[Tripplebeards]

No just flaring the mouth. Explains why hornady was giving away the bullets with a die purchase. I measured 2 and they were both the same.

Hornady said if I send what’s left back and they check out bad they would send me 2 boxes in exchange.

[Itzdapig]

I just measured some from a box I have here, they are .430 all the way to the cannelure.

[Wis Tom]

I just measured a few out of each of my 4 boxes, of Hornady 44 mag bullets, and all are .430 all the way up to the cannelure. The bullets, I suspect, would be your problem. My cast are all between .429 and .430, depending on the different molds I use.